Closed center valve control system

ABSTRACT

An actuator system wherein on-off solenoid operated valves of piston type actuators are controlled so as to establish a dead band. The control valves are energized by time variable width pulses generated by modulator means responsive to input position information and threshold bias signals which determine the dead band.

O Unlted States Patent [1 1 1111 3,763,745 Andersen Oct. 9, 1973 [54]CLOSED CENTER VALVE CONTROL 3,521,535 7/1970 Oelrich 91/186 SYSTEM3,561,326 2/1971 Cassaday 91/363 R 3,664,234 5 1972 S 1 [75] Inventor:Joseph J. Andersen, Bristol, Conn. I muons 9 [363 R [73] Assignee:Chandler Evans Inc., West Hart- Primary ExaminerPaul E. Maslousky Vford, Conn V I rm W Attorney-David S. Fishman et al. [22] Filed: Jan.28, 1972 211 Appl. No.: 221,652 [57] ABSTRACT An actuator system whereinon-off solenoid operated valves of piston type actuators are controlledso as to 5" K s 331 188 establish a dead band. The control valves areener- 58] Fleid kggg l 91 47 363 R gized by time variable width pulsesgenerated by mod- ,5 ulator means responsive to input positioninformation and threshold bias signals which determine the dead [56]References Cited band UNITED STATES PATENTS 13 Claims, 8 Drawing Figures3,266,378 8/1966 Shaw 91/363 R osc CCW

THRESHOLD INPUT COMMAND MODULATOR CW THRESHOLD .2. 7 iii 7 PATENTEDUBT91915 3.763.745

SHEET EM 3 A AM CW THRESHOLD 2c a AVAV V v KCCW THRESHOLD PATENTEUUET91m 3.763.745

SHEET 30F 3 FIGS I YIKGROUND OF THE INVENTION ield of the Invention Thepresent invention relates to the positioning of movable members. Morespecifically, the present invention is directed to fluidic actuators andload position control systems including such actuators. Accordingly, thegeneral objects of the present invention are to provide novel andimproved methods and apparatus of such character.

2. Description of the Prior Art While not limited thereto in itsutility, the actuator system of the present invention is particularlywell suited for use in environments wherein very severe reliability,weight and space requirements are imposed on the means for positioningmovable members. Thus, by way of example and for purposes of explanationonly, the invention may be considered in the environment of a steeringfin actuator for a missile.

Position control systems employing fluidic actuators, including thosewhich utilize a self-contained pressurized gas source to supply powerfor moving a load, are well known in the art. Those prior art controlsystems which are of the time modulated type have been found to beparticularly well suited for use in quasicontinuous open and also inclosed loop controls where operating fluid consumption as well as weightand space limitations are severe. Time modulated pneumatic actuatorsystems are disclosed in U.S. Pat. Nos. 3,430,536, 3,516,331, and3,521,535; all of said patents being assigned to the assignee of thepresent invention and being incorporated herein by reference.

SUMMARY OF TI-IE INVENTION The present invention constitutes anadvancement over the actuator systems of the prior art in that itcomprises a closed center valve control systemwith improved power fluidconsumption characteristics. The actuator system of the presentinvention also offers improvements in stability, particularly whencompared toprior art differential area piston devices, due to itssymmetry of operation. In a preferred embodiment, the present inventionmay be considered a time modulated, closed center valve control systemwherein on/off solenoid operated ball poppet valves are utilized toposition associated and interconnected piston type actuators. Thesolenoids of the valves are energized by signals generated by controlelectronics which provides time variable width pulses. These pulses aredelivered to the solenoids in such a manner as to selectively controlflow to the appropriate actuator to thereby control the differentialpressure across the actuator system.

In accordance with a particularly novel feature of the present inventionthe mechanical output of the actuator system is derived from a pair ofactuator pistons in such a manner that clockwise or counterclockwisedirected torque may be produced. The pistons of the actuators aredisposed in respective cylinders and are mechanically interconnected viaa rotatable output mechanism. A solenoid operated ball poppet valve isassociated with each actuator and, under the comma'nd of time modulatedenergization signals provided by the control electronics, the valvescontrol the pressure as measured at the first sides of the two pistons;i.e., the differential pressure across the actuator system. Controlpressure from a source is normally applied to the first faces of bothpistons. In response to the energization signals one of the valves willbe momentarily opened thereby venting the associated cylinder andpermitting the source pressure acting on the first face of the piston inthe other cylinder to push the actuator system output mechanism in theappropriate direction.

In accordance with a further novel feature of the invention thresholdcontrol signals may be applied to the control electronics in theinterest of providing a dead zone in the valve operating characteristic.The existence and magnitude of the dead zone will determine the degreeof closed centerness of the control valves. In steady-state operationthe coexistence of a dead zone which encompasses the region in whicheach valve would otherwise operate will result in a closed valvecondition with virtually zero leakage flow.

In accordance with a further feature of the present inventionsteady-state leakage flow may be totally eliminated by incorporation ofa power source shut-off valve between the source and actuators. Use ofthe pressure shut-off valve permits gas pressure to be entrapped in thecylinders of the actuators whereby steady-state torque may be maintainedwith zero leakage flow.

BRIEF DESCRIPTION OF THE DRAWING The present invention may be betterunderstood and its numerous objects and advantages will become apparentto those skilled in the art by reference to the accompanying drawing inwhich:

FIG. 1 is a schematic representation of a first embodiment of thepresent invention;

FIGS. 2A through 2F depict the waveforms of signals which appear atvarious points in the electronic control circuitry of the presentinvention; and

FIG. 3 is a plot of the functional relationship between slewing rateflow and input error signal magnitude and also of the functionalrelationship between the differential pressure developed across theactuator system of the FIG. 1 embodiment and input error signalmagnitude.

i DESCRIPTION OF PREFERRED EMBODIMENT With'reference now to FIG. 1, anactuator system having an output member 10 capable of-clockwise andcounterclockwise motion is depicted schematically. Output member 10 willtypically be coupled, by a drive shaft or other mechanical linkage meansnot shown, to a load such as a missle steering fin. The arms 12 and 14of actuator 10 are respectively mechanically coupled to pistons 16 and18. Pistons l6 and 18 are positioned in respective cylinders 20 and 22of a pair of pneumatic actuators. It is to be noted that the twoactuators have been shown separately in FIG. 1 in the interest offacilitating understanding of the invention. In actual practice,however, both of cylinders 20 and 22 will be formed within the samehousing; the housing for the pneumatic subsystem of the actuator systemof the invention being indicated generally at 24.

The housing 24 is provided with internal passages by which fluidcommunication is established between a source of pressurized fluid,typically gas, and cylinders 20 and 22 at first sides of pistons 16 and18. In the disclosed embodiment the pressurized source includes a bottleor tank 26 and a T-conduit 28 which extends between tank 26 andappropriate inlets in housing 24. 1t

is to be observed that the base leg of T-conduit 28 includes a shut-offvalve 30 which is optional equipment. The purpose of valve 30 will bedescribed in detail below. It is also to be noted that tank 26 willtypically include a pressure regulator whereby the pressure applied toeach of cylinders 20 and 22 will remain constant throughout the usefuloperating life of a selfcontained source.

Immediately downstream of each of the discharge ends of T-conduit 28 thehousing 24 is provided with a valve chamber. Check valves 32 and 34 areinstalled in these chambers and prevent the flow of gas from respectiveof cylinders 20 and 22 back toward the source 26; both check valvesbeing shown in the abnormal closed position. The chambers includingvalves 32 and 34 are respectively connected to further valve chambers inhousing 24 via passages 36 and 38. These further valve chambers,indicated respectively at 40 and 42, are provided with oppositelydisposed seats; first seats being formed at the inlets from passages 36and 38 and second seats being formed at the entrance to vent ports 44and 46. Continuous communication between chambers 40 and 42 and theirassociated cylinders 20 and 22 is provided respectively by passages 48and 50.

The balls of respective of a pair of solenoid operated ball poppetvalves, indicated generally at 52 and 54, are positioned within chambers40 and 42 and in cooperating relationship with the opposed inlet andvent ports to the chamber. Valves 52 and 54 may be of the type clearlyshown in FIG. 4 of referenced U.S. Pat. No. 3,521 ,535. The balls ofvalves 52 and 54 close respective vent passages 44 and 46 with theactuators in their quiescent condition as shown. Actuation of the valvesbetween the positions shown and sealing relationship to the seat formedat the inlet ends of respective passages 36 and 38 is achieved bymagnetically actuated guideplunger assemblies. The magnetic fields foroperating valves 52 and 54 are generated by the application of controlsignals to respective solenoids 72 and 74.

The control electronics for the embodiment of FIG. 1 comprises means forgenerating time modulated energization signals for solenoids 72 and 74.These energization signals are employed to control respective switches76 and 78 which in turn control the application of direct current from asource, not shown, to the solenoids. The time modulated signals aregenerated by modulators 80 and 82. Modulator 82 is the clockwisemodulator and generating energization signals for valve 54 in order toposition piston 18. Modulator 80 is the counterclockwise control signalsource and provides energization signals which ultimately control theoperation of valve 52 in the interest of moving piston 16. In thepreferred embodiment three input signals are applied to each ofmodulators 80 and 82. The first input signal is the time varying carriervoltage generated by oscillator 84. A preferred linear output waveformwhich may be provided by oscillator 84 is depicted in FIG. 2B. Inaddition to the carrier signal, each of modulators 80 and 82 have, inthe disclosed closed loop embodiment, a position error signal, asrepresented by FIG. 2A, applied thereto. The position error signal willbe a d.c. voltage level commensurate with any deviation between theactual position of the load connected to output member and the desiredload position. Modulator 80 has, in addition, a counterclockwisethreshold signal, in the form of a d.c. bias, applied thereto. Modulator82 additionally has a clockwise threshold bias applied thereto. Thecounterclockwise and clockwise threshold bias signals are respectivelyindicated in FIGS. 2C and 2E with appropriate labeling.

In a closed loop application position transducer means 86 ismechanically coupled to actuator 10. Transducer 86 may, for example, beone or a pair of potentiometers having their wiper arms mechanicallycoupled to member 10 for movement therewith. The signal appearing on thewiper arm or arms of the transducer means 86 will be compared with aninput command signal in a summing circuit 88 to generate the positionerror signal applied to modulators and 82.

Modulators 80 and 82 will typically comprise high gain operationalamplifiers which operate as switches in the manner well known in theart.

From consideration of FIG. 2 it may be seen that the position errorsignal, waveform 2A, is summed with the carrier and in effect causes thecarrier to be shifted. The shifted carrier signals are depicted in FIGS.2C and 2E and the corresponding modulator output pulse trains aredepicted in FIGS. 2D and 2F. It may be seen that, when the shiftedcarrier exceeds the threshold bias, switch energization pulses will begenerated; the pulses having a width commensurate with the time duringeach cycle of the carrier that the carrier exceeds the threshold level.These time variable energization pulses control the operation of valves52 and 54. Thus, to summarize, the carrier oscillator 84 generates atime varying signal which is applied to the input of each of pulseproducer modulators 80 and 82. The amplitude of the carrier signalwaveform is such that it alone will not switch either of the modulatorsfrom its normally off command condition. The application of an inputcommand or position error signal will cause the carrier waveform to beshifted whereby its amplitude would normally be sufficient to cause themodulators to begin producing output pulses. A dead zone of no pulsegeneration is, however, created by the application of threshold biassignals to the modulators. By introducing a command signal of sufficientmagnitude; i.e., greater than the established dead zone; a'time variablewidth switching or energization pulse will be generated. Through the useof a triangular waveform carrier, although other waveforms are possible,a linear variation in the modulator output pulse width with error orcommand signal magnitude is obtained.

The power level of the time variable command pulses from modulators 80and 82 are amplitude modulated by the solenoid switches 76 and 78 inorder to drive the solenoid operated ball valves. The function of eachof the solenoid operated ball valves, acting in response to timevariable command pulses generated by the modulators, is to normallyreside in a quiescent or inactive state while the time variable widthpulses are within" the dead zone. In this condition, the ball poppets ofeach valve will be held, by full actuator chamber pressure, on thedischarge seat to prevent direct flow from the actuator chambers throughvent ports 44 and 46. At this time the reverse flow check valves 32 and34 will prevent flow from the actuator chambers back to the supply.Accordingly, with the valves 52 and 54 in the quiescent state, theworking gas will be entrapped in the actuator chambers therebydeveloping maximum stiffness to disturbing loads applied to the actuatorsystem. In addition, essentially all leakage from the chambers 20 and 22is eliminated and minimal pneumatic power is wasted from the source.

The operating function of the solenoid operated ball valves, for activecommand wherein the modulated carrier is at least partly outside of thedead zone, is to develop a time modulated output. In the time modulatedoutput mode the valve operation converts the pneumatic supply pressureinto a substantially steady-state force which acts on output member todisplace a compliant load. Thus, in response to a command which exceedsthe threshold bias, one of valves 52 or 54 will be periodically openedin accordance with a pulse width modulated control signal and pressurewill be vented from the one of cylinders and 22 associated with theoperated valve while source pressure will be maintained in the othercylinder thereby resulting in pushing of the piston in said othercylinder and rotational motion of member 10. It will be understood that,if the load on the actuator is a force or torque, one of the valves willbe continuously pulsed until the command falls within the dead zone.

Operation of the present invention may be further understood byreference to FIG. 3. In this figure a plot of differential load pressureapplied to the actuator for an input command signal of varying magnitudeis depicted as a solid line. The dominant feature shown in FIG. 3 is thedead zone that has been created in the pressure characteristics byapplication of the threshold bias signals. The broken line in FIG. 3 isa plot of the slewing rate flow characteristics of the actuator vrssignal command; ln steady-state operation the coexistence of both ofthese operating dead zones will result in a closed valve condition withnearly zero leakage flow.

Leakage may be further reduced, and in fact virtually eliminated, forall conditions of differential pressure or developed torque through theuse of the power source shut-off valve 30. Valve 30 may, for example, bea twoway solenoid controlled valve similar to valves 52 and 54. Valve 30may be operated to close off communication between source 26 and theactuators and to vent the actuator supply conduit 28 whereby existinggas pressures will be entrapped, by the closed control valves 52 and 54and the reverse flow check valves 32 and 34, in the actuator cylindersto maintain steadystate torque with zero leakage flow. Valve 30 may becontrolled by the position error signal via a comparator circuit 90.Circuit 90 will establish a dead zone commensurate with that imposed onmodulators 80 and 82.

While a preferred embodiment has been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:

l. A fluid driven actuator system comprising:

first piston means, said first piston means including a first drivepiston disposed for motion in a cylinder; means coupling a first side ofsaid first drive piston to a movable load;

means for supplying pressurized fluid to said first piston meanscylinder at the second side of said first drive piston;

means for venting fluid from said first piston means cylinder at saidsecond side of said first drive piston;

a first normally deenergized and closed solenoid operated control valvefor sealing said venting means, said first solenoid operated controlvalve opening said venting means to reduce the pressure in said 5 firstpiston means cylinder when in the energized condition to thereby permitmovement of said first drive piston in a first direction;

modulator means responsive to a time varying signal having a mean valuewhich is a function of load position error and to a threshold biassignal for generating a pulse width modulated valve control energizationsignal whenever said time varying signal exceeds said threshold biassignal whereby a dead zone for said piston means is created; and

means for coupling valve control energization signals generated by saidmodulator means to the solenoid of said first control valve.

2. The apparatus of claim 1 wherein said means for supplying pressurizedfluid comprises:

20 a pressurized fluid source; and

check valve means interposed between said fluid source and said firstpiston means cylinder to prevent reverse flow from said cylinder to saidsource, said check valve means cooperating with said first control valveto entrap operating fluid in said first piston means cylinder with saidcontrol valve in the normally closed condition.

3. The apparatus of claim 2 further comprising:

shut-off valve means for disrupting communication between thepressurized fluid source and said check valve means whereby fluid may beentrapped in said piston means cylinder with the piston in any position.

4. A fluid driven actuator system comprising:

first piston means, said first piston means including a first drivepiston disposed for motion in a cylinder;

means coupling a first side of said first drive piston to a movableload;

second piston means, said second piston means including a second drivepiston disposed for motion in a cylinder;

means coupling a first side of said second drive piston to the movableload;

means for supplying pressurized fluid to said first and second pistonmeans cylinders at the second sides of said drive pistons;

first control valve means for adjusting the pressure in said firstpiston means cylinder at the second side of said first drive piston tovary the position of said first drive piston;

second control valve means for adjusting the pressure in said secondpiston means cylinder at said second side of said second drive piston tovary the position of said second drive piston;

means for operating said first control valve means in response to valvecontrol energization signals;

means for operating said second control valve means in response to valvecontrol energization signals;

oscillator means for generating a time varying carrier signal;

modulator means responsive to said time varying sig-- nal provided bysaid oscillator means and to signals commensurate with position errorsof the movable load for generating a time varying signal having a meanvalue which is a function of load position error, said modulator meansbeing further responsive to clockwise and counterclockwise thresholdbias signals, said modulator means generating pulse width modulatedvalve control energization signals whenever said generated time varyingsignal exceeds one of said threshold bias signals whereby a dead zonefor said piston means is created;

means for coupling valve control energization signals generated by saidmodulator means and commensurate with position errors in a firstdirection to said first control valve operating means; and

means for coupling valve control energization signals generated by saidmodulator means and commensurate with position errors in a seconddirection to said second control valve operating means.

5. The apparatus of claim 4 wherein said modulator means comprises:

means responsive to signals commensurate with the actual and desiredpositions of the load for generating first and second error signalsrespectively commensurate with clockwise and counterclockwise positionerrors;

first amplifier means connected to said oscillator means and saidposition error signal generating means and responsive to a signalcommensurate with a clockwise position error and to said time varyingcarrier signal, said first amplifier means further being responsive to aclockwise threshold bias signal and generating a pulse width modulatedvalve energization signal for application to said first control valvemeans whenever the sum of said clockwise position error and time varyingsignals exceeds said clockwise threshold bias signal; and

second amplifier means connected to said oscillator means and saidposition error signal generating means and responsive to a signalcommensurate with counterclockwise load position error and to said timevarying carrier signal, said second amplifier means further beingresponsive to a counterclockwise threshold bias signal and generating apulse width modulated valve energization signal for application to saidsecond control valve means whenever the sum of said counterclockwiseposition error and time varying signals exceeds said counterclockwisebias signal.

6. The apparatus of claim 4 wherein said control valve means eachcomprises:

means for venting fluid from the associated piston means cylinder; and

normally closed and deenergized solenoid operated valve meanscooperating with said venting means to prevent escape of pressurizedfluid from the associated piston means cylinder in the closed condition,energization of said valve means opening said venting means whereby areduction in the associated cylinder pressure results.

7. the apparatus of claim 6 wherein said pressurized fluid supplyingmeans comprises:

a pressurized fluid source;

conduit means for coupling said source to said first piston meanscylinder, said conduit means including first check valve means forpreventing reverse flow of fluid from said first piston means cylindertoward the source, said first check valve means cooperating with saidfirst control valve means for entrapping operating fluid in said firstpiston means cylinder when said first control valve means is in theclosed position; and

second conduit means for coupling said source to said second pistonmeans cylinder, said second conduit means including second check valvemeans for preventing reverse flow of fluid from said sec- 0nd pistonmeans cylinder toward the source, said second check valve meanscooperating with said second control valve means for entrappingoperating fluid in said second means cylinder when said second controlvalve means is in the closed position. 8. The apparatus of claim 7wherein said modulator means comprises:

means responsive to signals commensurate with the actual and desiredpositions of the load for generating first and second error signalsrespectively commensurate with clockwise and counterclockwise positionerrors; first amplifier means connected to said oscillator means andsaid position error signal generating means and responsive to a signalcommensurate with a clockwise position error and to said time varyingcarrier signal, said first amplifier means further being responsive to aclockwise threshold bias signal and generating a pulse width modulatedvalve energization signal for application to said first control valvemeans whenever the sum of said clockwise position error and time varyingsignals exceeds said clockwise threshold bias signal; and secondamplifier means connected to said oscillator means and said positionerror signal generating means and responsive to a signal commensuratewith counterclockwise load position error and to said time varyingcarrier signal, said second amplifier means further being responsive toa counterclockwise threshold bias signal and generating a pulse widthmodulated valve energization signal for application to said secondcontrol valve means whenever the sum of said counterclockwise positionerror and time varying signals exceeds said counterclockwise biassignal. 9. The apparatus of claim 8 further comprising: shut-off valvemeans for disrupting communication between said source and said checkvalve means and for venting said conduit means whereby fluid may beentrapped in said piston means cylinders with the piston in anyposition. 10. The apparatus of claim 9 wherein said shut-off valve meanscomprises:

a solenoid controlled valve; and means responsive to a signalcommensurate with load position error for generating control signals forsaid shut-off valve means solenoid controlled valve, said signalgenerating means establishing a dead band commensurate with the deadzone created by said modulating means. 11. The apparatus of claim 7further comprising: shut-off valve means for disrupting communicationbetween said source and said check valve means and for venting saidconduit means whereby fluid may be entrapped in said piston meanscylinders with the piston in any position. 12. The apparatus of claim 11wherein said shut-off valve means comprises:

a solenoid controlled valve; and means responsive to a signalcommensurate with load position error for generating control signals forsaid shut-off valve means solenoid controlled valve, said position errorfor generating control signals for said shut-off valve means solenoidcontrolled valve, said signal generating means establishing a dead bandcommensurate with the dead zone created by said modulating means.

1. A fluid driven actuator system comprising: first piston means, saidfirst piston means including a first drive piston disposed for motion ina cylinder; means coupling a first side of said first drive piston to amovable load; means for supplying pressurized fluid to said first pistonmeans cylinder at the second side of said first drive piston; means forventing fluid from said first piston means cylinder at said second sideof said first drive piston; a first normally deenergized and closedsolenoid operated control valve for sealing said venting means, saidfirst solenoid operated control valve opening said venting means toreduce the pressure in said first piston means cylinder when in theenergized condition to thereby permit movement of said first drivepiston in a first direction; modulator means responsive to a timevarying signal having a mean value which is a function of load positionerror and to a threshold bias signal for generating a pulse widthmodulated valve control energization signal whenever said time varyingsignal exceeds said threshold bias signal whereby a dead zone for saidpiston means is created; and means for coupling valve controlenergization signals generated by said modulator means to the solenoidof said first control valve.
 2. The apparatus of claim 1 wherein saidmeans for supplying pressurized fluid comprises: a pressurized fluidsource; and check valve means interposed between said fluid source andsaid first piston means cylinder to prevent reverse flow from saidcylinder to said source, said check valve means cooperating with saidfirst control valve to entrap operating fluid in said first piston meanscylinder with said control valve in the normally closed condition. 3.The apparatus of claim 2 further comprising: shut-off valve means fordisrupting communication between the pressurized fluid source and saidcheck valve means whereby fluid may be entrapped in said piston meanscylinder with the piston in any position.
 4. A fluid driven actuatorsystem comprising: first piston means, said first piston means includinga first drive piston disposed for motion in a cylinder; means coupling afirst side of said first drive piston to a movable load; second pistonmeans, said second piston means including a second drive piston disposedfor motion in a cylinder; means coupling a first side of said seconddrive piston to the movable load; means for supplying pressurized fluidto said first and second piston means cylinders at the second sides ofsaid drive pistons; first control valve means for adjusting the pressurein said first piston means cylinder at the second side of said firstdrive piston to vary the position of said first drive piston; secondcontrol valve means for adjusting the pressure in said second pistonmeans cylinder at said second side of said second drive piston to varythe position of said second drive piston; means for operating said firstcontrol valve means in response to valve control energization signals;means for operating said second control valve means in response to valvecontrol energization signals; oscillator means for generating a timevarying carrier signal; modulator means responsive to said time varyingsignal provided by said oscillator means and to signals commensuratewith position errors of the movable load for generating a time varyingsignal having a mean value which is a function of load position error,said modulator means being further responsive to clockwise andcounterclockwise threshold bias signals, said modulator means generatingpulse width modulated valve control energization signals whenever saidgenerated time varying signal exceeds one of said threshold bias signalswhereby a dead zone for said piston means is created; means for couplingvalve control energization signals generated by said modulator means andcommensurate with position errors in a first direction to said firstcontrol valve operating means; and means for coupling valve controlenergization signals generated by said modulator means and commensuratewith position errors in a second direction to said second control valveoperating means.
 5. The apparatus of claim 4 wherein said modulatormeans comprises: means responsive to signals commensurate with theactual and desired positions of the load for generating first and seconderror signals respectively commensurate with clockwise andcounterclockwise position errors; first amplifier means connected tosaid oscillator means and said position error signal generating meansand responsive to a signal commensurate with a clockwise position errorand to said time varying carrier signal, said first amplifier meansfurther being responsive to a clockwise threshold bias signal andgenerating a pulse width modulated valve energization signal forapplication to said first control valve means whenever the sum of saidclockwise position error and time varying signals exceeds said clockwisethreshold bias signal; and second amplifier means connected to saidoscillator means and said position error signal generating means andresponsive to a signal commensurate with counterclockwise load positionerror and to said time varying carrier signal, said second amplifiermeans further being responsive to a counterclockwise threshold biassignal and generating a pulse width modulated valve energization signalfor application to said second control valve means whenever the sum ofsaid counterclockwise position error and time varying signals exceedssaid counterclockwise bias signal.
 6. The apparatus of claim 4 whereinsaid control valve means each comprises: means for venting fluid fromthe associated piston means cylinder; and normally closed anddeenergized solenoid operated valve means cooperating with said ventingmeans to prevent escape of pressurized fluid from the associated pistonmeans cylinder in the closed condition, energization of said valve meansopening said venting means whereby a reduction in the associatedcylinder pressure results.
 7. the apparatus of claim 6 wherein saidpressurized fluid supplying means comprises: a pressurized fluid source;conduit means for coupling said source to said first piston meanscylinder, said conduit means including first check valve means forpreventing reverse flow of fluid from said first piston means cylindertoward the source, said first check valve means cooperating with saidfirst control valve means For entrapping operating fluid in said firstpiston means cylinder when said first control valve means is in theclosed position; and second conduit means for coupling said source tosaid second piston means cylinder, said second conduit means includingsecond check valve means for preventing reverse flow of fluid from saidsecond piston means cylinder toward the source, said second check valvemeans cooperating with said second control valve means for entrappingoperating fluid in said second means cylinder when said second controlvalve means is in the closed position.
 8. The apparatus of claim 7wherein said modulator means comprises: means responsive to signalscommensurate with the actual and desired positions of the load forgenerating first and second error signals respectively commensurate withclockwise and counterclockwise position errors; first amplifier meansconnected to said oscillator means and said position error signalgenerating means and responsive to a signal commensurate with aclockwise position error and to said time varying carrier signal, saidfirst amplifier means further being responsive to a clockwise thresholdbias signal and generating a pulse width modulated valve energizationsignal for application to said first control valve means whenever thesum of said clockwise position error and time varying signals exceedssaid clockwise threshold bias signal; and second amplifier meansconnected to said oscillator means and said position error signalgenerating means and responsive to a signal commensurate withcounterclockwise load position error and to said time varying carriersignal, said second amplifier means further being responsive to acounterclockwise threshold bias signal and generating a pulse widthmodulated valve energization signal for application to said secondcontrol valve means whenever the sum of said counterclockwise positionerror and time varying signals exceeds said counterclockwise biassignal.
 9. The apparatus of claim 8 further comprising: shut-off valvemeans for disrupting communication between said source and said checkvalve means and for venting said conduit means whereby fluid may beentrapped in said piston means cylinders with the piston in anyposition.
 10. The apparatus of claim 9 wherein said shut-off valve meanscomprises: a solenoid controlled valve; and means responsive to a signalcommensurate with load position error for generating control signals forsaid shut-off valve means solenoid controlled valve, said signalgenerating means establishing a dead band commensurate with the deadzone created by said modulating means.
 11. The apparatus of claim 7further comprising: shut-off valve means for disrupting communicationbetween said source and said check valve means and for venting saidconduit means whereby fluid may be entrapped in said piston meanscylinders with the piston in any position.
 12. The apparatus of claim 11wherein said shut-off valve means comprises: a solenoid controlledvalve; and means responsive to a signal commensurate with load positionerror for generating control signals for said shut-off valve meanssolenoid controlled valve, said signal generating means establishing adead band commensurate with the dead zone created by said modulatingmeans.
 13. The apparatus of claim 3 wherein said shut-off valve meanscomprises: a solenoid controlled valve; and means responsive to a signalcommensurate with load position error for generating control signals forsaid shut-off valve means solenoid controlled valve, said signalgenerating means establishing a dead band commensurate with the deadzone created by said modulating means.